1. Field of the Invention
The invention relates to a directional coupler.
2. Related Technology
Directional couplers of this kind are known, for example, from Meinke/Grundlach, Taschenbuch der Hochfrequenztechnik [Handbook of High-Frequency Technology], 5th edition, pages L29 to L34. An ideal separation of the forward and returning waves in this context is possible only with directional couplers, which allow for a propagation of TEM waves. Hitherto, this has been possible only with directional couplers in coaxial line technology. Directional couplers in microstripline or coplanar line technology do not allow a propagation of pure TEM waves. Moreover, directional couplers in coaxial line technology are relatively complex in structure. However, the relatively-simpler structure of directional couplers in microstripline or coplanar line technology has the disadvantage that it does not allow a pure TEM-wave propagation, and, accordingly, the phase constants of the even and odd modes, which are so important for the function of a directional coupler, are not identical.